Fingerprint sensor with different measurement circuitry configurations

ABSTRACT

A fingerprint sensor comprising a plurality of electrically conductive sensing structures arranged in a sensing plane for capacitively sensing a fingerprint of a finger; and measurement circuitry coupled to the plurality of electrically conductive sensing structures for providing measurement signals indicative of a capacitive coupling between each sensing structure in the plurality of electrically conductive sensing structures and the finger, wherein the measurement circuitry is arranged to: provide, using a first measurement circuitry configuration, a first set of measurement signals from a first set of sensing structures; and provide, using a second measurement circuitry configuration different from the first measurement circuitry configuration, a second set of measurement signals from a second set of sensing structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Swedish Patent Application No.2051062-4, filed on Sep. 10, 2020. The disclosure of the aboveapplication is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a fingerprint sensor to be included ina fingerprint sensor arrangement comprising dielectric material having anon-uniform thickness profile defining a topography of a fingerreceiving surface of the fingerprint sensor arrangement. The presentinvention also relates to a fingerprint sensor arrangement, and to anelectronic device.

BACKGROUND OF THE INVENTION

Biometric systems are widely used as means for increasing theconvenience and security of personal electronic devices, such as mobilephones etc. Fingerprint sensing systems, in particular, are now includedin a large proportion of all newly released personal communicationdevices, such as mobile phones.

Many electronic devices have various curved surfaces for providing animproved user experience. It would be desirable to provide for improvedintegration of fingerprint sensing capability in such electronicdevices.

SUMMARY

In view of the above, it is an object of the present invention toprovide for improved integration of fingerprint sensing capability in anelectronic device having at least one curved surface portion.

According to a first aspect of the present invention, it is thereforeprovided a fingerprint sensor to be included in a fingerprint sensorarrangement comprising dielectric material having a non-uniformthickness profile defining a topography of a finger receiving surface ofthe fingerprint sensor arrangement, the fingerprint sensor comprising: aplurality of electrically conductive sensing structures arranged in asensing plane for capacitively sensing a fingerprint of a finger placedon the finger receiving surface of the fingerprint sensor arrangement;and measurement circuitry coupled to the plurality of electricallyconductive sensing structures for providing measurement signalsindicative of a capacitive coupling between each sensing structure inthe plurality of electrically conductive sensing structures and thefinger, wherein the measurement circuitry is arranged to: provide, usinga first measurement circuitry configuration, a first set of measurementsignals from a first set of sensing structures in the plurality ofelectrically conductive sensing structures, arranged to be covered by athickness of the dielectric material within a first thickness range; andprovide, using a second measurement circuitry configuration differentfrom the first measurement circuitry configuration, a second set ofmeasurement signals from a second set of sensing structures in theplurality of electrically conductive sensing structures, arranged to becovered by a thickness of the dielectric material within a secondthickness range, only including greater thicknesses than the firstthickness range.

It should be noted that the term “measurement circuitry configuration”may include either or both of a physical circuitry configuration and acircuitry setting.

The present invention is based on the general realization that it wouldbe beneficial to enable a non-flat topography of a finger receivingsurface, in an electronic device using a capacitive fingerprint sensorin which the sensing structures are arranged in a sensing plane. Thepresent inventors have further realized that the sensing performance ofa fingerprint sensor in such a configuration can be improved by usingdifferent measurement circuitry configurations for different parts ofthe sensing plane, or in other words, different measurement circuitryconfigurations for different sets of sensing structures, arranged to becovered by different thicknesses of dielectric material.

Hereby, a more uniform signal strength baseline can be achieved acrossthe sensing plane, which in turn provides for improved fingerprint imagequality, which can be translated to improved biometric authenticationperformance of the electronic device in which the fingerprint sensoraccording to embodiments of the present invention is integrated.

Depending on the design of the electronic device into which thefingerprint sensor should be integrated, the first and secondmeasurement circuitry configurations may be used for different parts ofthe sensing plane of the fingerprint sensor. For a fingerprint sensor tobe integrated in a convex surface portion of an electronic device, thesecond set of sensing structures, for which the second measurementcircuitry configuration is used, may be more centrally located than thefirst set of sensing structures, for which the first measurementcircuitry configuration is used. For an electronic device in which thefinger receiving surface should be concave, the first set of sensingstructures may be more centrally located than the second set of sensingstructures, etc.

The measurement circuitry may be arranged to use additional, differentmeasurement circuitry configurations for additional sets of sensingstructures.

According to various embodiments, furthermore, each sensing structure inthe first set of sensing structures may cover an area in the sensingplane being within a first area range; and each sensing structure in thesecond set of electrically conductive sensing structures may cover anarea in the sensing plane being within a second area range onlyincluding areas greater than first area range.

Sensing structures covering areas within different area ranges providesa further tool for improving the sensing performance of a fingerprintsensor with a sensing plane integrated in an electronic device with anon-planar finger receiving surface. In particular, the different areasof the sensing structures may be selected for achieving an even moreuniform signal strength baseline across the sensing plane, which in turnprovides for improved fingerprint image quality, which can be translatedto improved biometric authentication performance of the electronicdevice in which the fingerprint sensor according to embodiments of thepresent invention is integrated. Furthermore, the performance of thefingerprint sensor can be made considerably less sensitive to commonmode interference, which may for example be introduced through thefinger (via the human body) or through the electronic device in whichthe fingerprint sensor is integrated. The common mode interference mayinclude low-frequency components (such as from the grid) and/orhigh-frequency components, which may originate from switching powersupplies or NFC-signaling etc. Since the sensing structures are first inthe sensing chain, mitigation of common mode interference at the sensingstructures, through the differently sized sensing structures, will bemore efficient at reducing the influence of common mode interferencethan efforts only involving other parts of the sensing chain.

The fingerprint sensor according to embodiments of the present inventionmay advantageously be included in a fingerprint sensor arrangement,further comprising dielectric material covering the sensing plane of thefingerprint sensor, the dielectric material having a non-uniformthickness profile defining the topography of the finger receivingsurface of the fingerprint sensor arrangement.

The dielectric material could, for example, be provided in the form of asuitably shaped window against which a planar surface of a fingerprintsensor according to embodiments of the present invention is pressed orglued. Alternatively, or in combination, the dielectric material may bemolded on the sensing surface of the fingerprint sensor, whereby it canpractically be ensured that there is no air gap between the sensingplane and the finger receiving surface.

According to a second aspect of the present invention, it is provided amethod of acquiring a fingerprint representation using a fingerprintsensor arrangement with a finger receiving surface to be touched by afinger, the fingerprint sensor arrangement comprising a plurality ofelectrically conductive sensing structures arranged in a sensing plane;measurement circuitry coupled to the plurality of electricallyconductive sensing structures for providing measurement signalsindicative of a capacitive coupling between each sensing structure inthe plurality of electrically conductive sensing structures and thefinger; and dielectric material covering the sensing plane, thedielectric material having a non-uniform thickness profile defining atopography of the finger receiving surface of the fingerprint sensorarrangement, the method comprising the steps of: acquiring, using afirst measurement circuitry configuration, a first set of measurementsignals from a first set of sensing structures in the plurality ofelectrically conductive sensing structures, covered by a thickness ofthe dielectric material within a first thickness range; and acquiring,using a second measurement configuration setting different from thefirst measurement circuitry, a second set of measurement signals from asecond set of sensing structures in the plurality of electricallyconductive sensing structures, covered by a thickness of the dielectricmaterial within a second thickness range, only including greaterthicknesses than the first thickness range.

In summary, embodiments of the present invention thus relate to afingerprint sensor comprising a plurality of electrically conductivesensing structures arranged in a sensing plane for capacitively sensinga fingerprint of a finger; and measurement circuitry coupled to theplurality of electrically conductive sensing structures for providingmeasurement signals indicative of a capacitive coupling between eachsensing structure in the plurality of electrically conductive sensingstructures and the finger, wherein the measurement circuitry is arrangedto: provide, using a first measurement circuitry configuration, a firstset of measurement signals from a first set of sensing structures; andprovide, using a second measurement circuitry configuration differentfrom the first measurement circuitry configuration, a second set ofmeasurement signals from a second set of sensing structures.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be describedin more detail, with reference to the appended drawings showing anexample embodiment of the invention, wherein:

FIG. 1 is an illustration of an exemplary electronic device comprising afingerprint sensor arrangement according to an embodiment of the presentinvention, in the form of a mobile phone;

FIG. 2 is an enlarged view of a portion of the electronic device in FIG.1;

FIGS. 3A-B are perspective views of the fingerprint sensor arrangementcomprised in the mobile phone in FIG. 1 and FIG. 2;

FIG. 4 is a partly opened perspective schematic illustration of thefingerprint sensor arrangement comprised in the mobile phone in FIG. 1and FIG. 2; and

FIG. 5 is a schematic cross-section view of the fingerprint sensorarrangement in FIG. 4;

FIG. 6 schematically illustrates a first embodiment of the fingerprintsensor according to the present invention;

FIG. 7 schematically shows a first example configuration of themeasurement circuitry in the fingerprint sensor in FIG. 6;

FIG. 8 schematically shows a second example configuration of themeasurement circuitry in the fingerprint sensor in FIG. 6;

FIGS. 9A-B schematically illustrate a second embodiment of thefingerprint sensor according to the present invention; and

FIGS. 10A-B schematically illustrate a third embodiment of thefingerprint sensor according to the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the present detailed description, various embodiments of thefingerprint sensor according to the present invention are mainlydescribed with reference to a fingerprint sensor arrangement in the formof a fingerprint sensor component including a semiconductor-basedcapacitive fingerprint sensor integrated circuit (IC). It should benoted that the fingerprint sensor according to embodiments of thepresent invention need not be overmolded by dielectric material, andthat the topography of the finger receiving surface could alternativelybe achieved using a separate part attached to or suitably arranged inrelation to the fingerprint sensor. Furthermore, the finger receivingsurface of the fingerprint sensor arrangement is mainly exemplified as aconvex surface. It should be noted that the present invention, asdefined by the claims, is not limited to any particular shape orconfiguration of the finger receiving surface, other than that it isdefined by a dielectric material having a non-uniform thickness profile.Accordingly, the finger receiving surface may be concave, partly convexand partly concave, or flat but inclined in relation to the sensingplane, etc. Moreover, the fingerprint sensor does not have to beelongated, but could be any other shape, such as square or round, etc.Furthermore, it should be understood that the mobile phone 1 in thefigures is only one example of an electronic device comprising thefingerprint sensor according to embodiments of the present invention.The fingerprint sensor according to embodiments of the present inventionmay advantageously be included in many other electronic devices,including, for example, computers, electronic watches and other gadgets,as well as smart cards, etc.

FIG. 1 schematically shows an electronic device, here in the form of amobile phone 1, comprising a device housing 3 and a fingerprint sensorarrangement, here in the form of fingerprint sensor component 5. As canbe seen in FIG. 1, the device housing 3 has a convex portion 7 with anopening 9. The fingerprint sensor component 5 is arranged in the opening9 and also exhibits a convex shape. The convex shape of the fingerprintsensor component 5 may substantially follow the convex shape of theconvex portion 7 of the device housing 3, at least at the opening 9.This is better seen in the partial enlargement in FIG. 2.

FIGS. 3A-B are perspective views of the fingerprint sensor component 5comprised in the mobile phone 1 in FIG. 1 and FIG. 2.

FIG. 3A is a perspective view showing a component top face 11 and a sidesurface 13 of the fingerprint sensor component 5. As can be seen in FIG.3A, the fingerprint sensor component 5 in this particular exampleconfiguration is elongated having a length l and a width w.

FIG. 3B is a perspective view showing a component bottom face 15 and theside surface 13 of the fingerprint sensor component 5. As can be seen inFIG. 3B, the fingerprint sensor component 5 has a component conductorpattern 17 on the component bottom face 15. In the embodiment of FIG.3B, the component conductor pattern 17 defines a land grid array with aplurality of component connection pads 19 (only one of these isindicated by a reference numeral in FIG. 3B to avoid cluttering thedrawing).

FIG. 4 is a partly opened perspective schematic illustration of thefingerprint sensor component 5 comprised in the mobile phone 1 in FIG.1, and shown in FIG. 3A and FIG. 3B.

Referring to FIG. 4, the fingerprint sensor component 5 comprises asubstrate 21, a fingerprint sensor die 23, and a molding material 25.The substrate 21 has a substrate top face 27 and a substrate bottom face29. The substrate top face 27 has a top face conductor pattern,including the bond pads 31 visible in FIG. 4, and (although not visiblein FIG. 4) the substrate bottom face 29 has a bottom face conductorpattern, which may constitute the component conductor pattern 17described above with reference to FIG. 3B. The fingerprint sensor die 23has a die top face 33, a die bottom face 35, and a side surface 37connecting the die top face 33 and the die bottom face 35. The die topface 33 includes a planar sensing surface 39. The die bottom face 35 ofthe fingerprint sensor die 23 is bonded to the substrate top face 27 ofthe substrate 21. As is schematically indicated in FIG. 4, thefingerprint sensor die 23 further comprises die connection pads 43,which are electrically connected to the bond pads 31 on the substratetop face 27 of the substrate 21. This electrical connection may beachieved using bond wires 45 as indicated in FIG. 4, or by any othersuitable connector known to the skilled person. The molding material 25covers the sensing surface 39 and the side surface 37 of the fingerprintsensor die 23, as well as a portion of the substrate top face 27 of thesubstrate 21 that is not covered by the fingerprint sensor die 23. As isschematically indicated in FIG. 4, the molding material 25 exhibits aconvex shape over the sensing surface 39 of the fingerprint sensor die23. As is schematically indicated in FIG. 4, the fingerprint sensorcomponent 5 may optionally additionally include a colored coating 47 ontop of the molding material 25.

FIG. 5 is a schematic cross-section view of the fingerprint sensorcomponent 5 in FIG. 4, of a section taken along the line A-A′ in FIG. 4.In addition to what has already been described above with reference toFIG. 4, FIG. 5 schematically shows a via 49 electrically connecting thetop face conductor pattern of the substrate 21 with the bottom faceconductor pattern 17 of the substrate 21.

FIG. 5 also indicates an advantageous configuration of the convex shapeof the molding material 25. As is indicated in FIG. 5, the top surfaceof the fingerprint sensor component 5 exhibits a convex shape with aradius R of curvature. The radius R of curvature of the fingerprintsensor component 5 may be adapted to substantially follow a radius ofcurvature of the convex portion 7 of the device housing 3 of theelectronic device 1.

FIG. 6 is a schematic cross-section view of a fingerprint sensorarrangement 5 comprising a fingerprint sensor 23 according to a firstembodiment of the present invention, and dielectric material 25 having anon-uniform thickness profile defining a topography, in this case aconvex topography, of a finger receiving surface 51 of the fingerprintsensor arrangement 5. The fingerprint sensor 23 comprises a plurality ofelectrically conductive sensing structures, here in the form of metalplates 41 arranged in a uniform array configuration in a sensingplane—the above-mentioned sensing surface 39—for capacitively sensing afingerprint of a finger 53 placed on the finger receiving surface 51 ofthe fingerprint sensor arrangement 5.

The plurality of electrically conductive sensing structures 41 includesa first set 41 a, a second set 41 b, and a third set 41 c of sensingstructures. the first set 41 a of sensing structures is arranged to becovered by a thickness of the dielectric material 25 within a firstthickness range d₁-d₂, the second set 41 b of sensing structures isarranged to be covered by thickness of the dielectric material 25 withina second thickness range d₃-d₄, only including greater thicknesses thanthe first thickness range d₁-d₂, and the third set 41 c of sensingstructures is arranged to be covered by thickness of the dielectricmaterial 25 within a third thickness range d₂-d₃, between the firstthickness range and the second thickness range.

The capacitive coupling between a sensing structure 41 of thefingerprint sensor 23 and a finger 53 placed on the finger receivingsurface 51 is a measure of the capacitance of the capacitor formed bythe sensing structure 41, the finger 53, and the dielectric material 25between the sensing structure 41 and the finger surface. In theconfiguration in FIG. 6, the maximum capacitance (resulting from a ridgeof the finger pattern in good contact with the finger receiving surface51) can be considered to be proportional to the surface area of thesensing structure 41 and inversely proportional to the distance betweenthe sensing structure and the finger surface.

As is schematically indicated in FIG. 6, the fingerprint sensor 23further comprises measurement circuitry 57 coupled to the sensingstructures 41 for providing measurement signals S_(m) indicative of thecapacitive coupling between the sensing structures 41 and the finger 53.

In FIG. 6, the measurement circuitry 57 is arranged to exhibit threedifferent measurement circuitry configurations 57 a c, schematicallyindicated as different blocks comprised in the total measurementcircuitry 57. It should be appreciated that this is only an illustrationto aid the understanding of this embodiment, and that the measurementcircuitry does not necessarily have different configurations indifferent parts thereof.

The measurement circuitry 57 in the fingerprint sensor 23 in FIG. 6 isthus arranged to provide, using the first measurement circuitryconfiguration 57 a, a first set of measurement signals resulting fromcapacitive coupling between the first set 41 a of sensing structures andthe finger 53, a second set of measurement signals resulting fromcapacitive coupling between the second set 41 b of sensing structuresand the finger 53, and a third set of measurement signals resulting fromcapacitive coupling between the third set 41 c of sensing structures andthe finger 53. The measurement signals S_(m) provided by the measurementcircuitry 57 may be a compilation of the first, second, and third setsof measurement signals.

The different configurations 57 a-c of the measurement circuitry 57 canbe permanent, or the measurement circuitry 57 can be controllable tochange measurement circuitry configuration for all of the sensingstructures 41, or for various sets of the sensing structures. Thisoptional controllability of the configuration of the measurementcircuitry is schematically indicated by the dashed arrow 59 in FIG. 6.Accordingly, the measurement circuitry 57 may be arranged to provide thefirst set of measurement signals, the second set of measurement signals,and the third set of measurement signals (if applicable) during afingerprint capture operation to allow formation of a fingerprintrepresentation Sm comprising the first set of measurement signals, thesecond set of measurement signals, and the third set of measurementsignals (if applicable). For instance, the measurement circuitry 57 maybe arranged to simultaneously provide the first set of measurementsignals, the second set of measurement signals, and the third set ofmeasurement signals (if applicable). Alternatively, a first fingerprintcapture operation may be performed using the first measurement circuitryconfiguration 57 a, and a second, subsequent, fingerprint captureoperation may be performed using the second measurement circuitryconfiguration 57 b, etc.

In a given measurement circuitry configuration, one or more propertiesof the measurement circuitry 57 may be adapted to the non-uniformthickness profile of the dielectric material 25 intended to cover thesensing plane 39 of the fingerprint sensor 23. For example, parameterssuch as offset, gain, or timing may be different for different sensingstructures 41, depending on the intended non-uniform thickness profileof the dielectric material 25. For instance, the offset and/or gainand/or measurement timing may be set to compensate for sensitivityvariations across the fingerprint sensor 23, resulting from thenon-uniform thickness profile.

The fingerprint sensor arrangement 5 may be comprised in a fingerprintsensing system, further comprising a fingerprint sensor controller thatis coupled to the fingerprint sensor 23 for controlling operation of thefingerprint sensor 23. The fingerprint sensor controller is notexplicitly shown in the drawings, but as will be clear to a person ofordinary skill in the art, the fingerprint sensor controller may beincluded in an electronic device, such as the mobile phone 1 in FIG. 1.The above-mentioned fingerprint sensing system may thus partly comprisecircuitry included in the electronic device. According to oneembodiment, the fingerprint sensor controller may be included in afingerprint sensor module that is integrated in the electronic device(mobile phone 1), and according to another embodiment, the functionalityof the fingerprint sensor controller may at least partly be realized bythe host controller of the electronic device.

In embodiments, the fingerprint sensor controller may be configured tocontrol, by means of control signals 59, the measurement circuitry 57 ofthe fingerprint sensor 23 to provide initial measurement signals usingan initial set of measurement circuitry configurations. Based on theinitial measurement signals, the fingerprint sensor controller maydetermine the first measurement circuitry configuration 57 a, the secondmeasurement circuitry configuration 57 b, and the third measurementconfiguration 57 c (where applicable), and control, by means of controlsignals 59, the measurement circuitry 57 to provide the first set ofmeasurement signals using the first measurement circuitry configuration57 a, the second set of measurement signals using the second measurementcircuitry configuration 57 b, and the third set of measurement signalsusing the third measurement configuration 57 c (where applicable).

FIG. 7 schematically shows a first example configuration of themeasurement circuitry 57 in the fingerprint sensor in FIG. 6, comprisingamplifier circuitry 63 and analog-to-digital converter (ADC) circuitry65. As is schematically indicated in FIG. 7, the amplifier circuitryreceives input from one or several sensing structures 41, and providesan amplified analog signal to the ADC circuitry 65. The ADC circuitry 65outputs digital measurement signals S_(m). To provide differentmeasurement circuitry configurations for different parts of thefingerprint sensor 23, the amplifier circuitry 63 and/or the ADCcircuitry 65 may exhibit different properties for sensing structures 41located in these different parts of the fingerprint sensor 23. To thatend, the amplifier circuitry 63 and/or the ADC circuitry 65 may bestructured differently for sensing structures 41 located in differentparts of the fingerprint sensor 23. Alternatively, the amplifiercircuitry 63 may be controllable using one or more external analogand/or digital control signals (illustrated by the dashed arrow 67)and/or the ADC circuitry 65 may be controllable using one or moreexternal control signals (illustrated by the dashed arrow 69).

FIG. 8 schematically shows a second example configuration of themeasurement circuitry in the fingerprint sensor in FIG. 6, in which theamplifier circuitry 63 is shown to comprise a charge amplifier 71, and asample-and-hold amplifier 73. The charge amplifier 71 may include anoperational amplifier 75, a feedback capacitor 77 and a reset switch 79.A first input 81 of the operation amplifier 75 may be connected to thesensing structure 41, and a second input 83 of the operation amplifier75 may be connected to ground, or to a controllable voltage sourcedepending on the overall configuration of the fingerprint sensorarrangement 5. The output 85 of the operational amplifier 75 isconnected to the sample-and-hold amplifier 73, and feedback coupled tothe first input 81 via the feedback capacitor 77 formed by the sensingstructure 41 and a feedback plate 87. The charge amplifier 71 can bereset by operating the reset switch 79 to short-circuit the first input81 and the output 85.

The amplifier circuitry 63 may operate in a, per se, known manner toprovide analog signals to the ADC circuitry 65. Properties of theamplifier circuitry 63 may, for example, be controlled using controlsignals affecting the offset of the operational amplifier (indicated bythe dashed arrow 89), the measurement time (by controlling the timing ofoperation of the reset switch 79), or using control signals affectingthe offset and/or gain of the sample-and-hold amplifier 73 (indicated bythe dashed arrow 91). Alternatively, or in combination, different chargeamplifiers 71 may permanently have different properties. For instance,the gain of the charge amplifier 71 is affected by the area of thefeedback plate 87.

Although it is indicated in FIG. 7 and FIG. 8 that one amplifier circuitis connected to a single sensing structure 41, it should be noted thatthis is not necessarily the case, and that, for example, one amplifiercircuit may be connected (selectively connectable) to a plurality ofsensing structures 41.

A second embodiment of the fingerprint sensor 23 according to thepresent invention will now be described with reference to FIGS. 9A-B,where FIG. 9A is a partial top view of the fingerprint sensor 23 withoutdielectric with non-uniform thickness covering the sensing surface 39,and FIG. 9B is a schematic cross-section view of a fingerprint sensorarrangement 5 comprising the fingerprint sensor 23 in FIG. 9A anddielectric material 25 having a non-uniform thickness profile defining atopography, in this case a convex topography, of a finger receivingsurface 51 of the fingerprint sensor arrangement 5.

Referring to FIGS. 9A-B, the fingerprint sensor 23 comprises a pluralityof electrically conductive sensing structures, here in the form of metalplates 41 arranged in a uniform array configuration in a sensingplane—the above-mentioned sensing surface 39—for capacitively sensing afingerprint of a finger 53 placed on the finger receiving surface 51 ofthe fingerprint sensor arrangement 5. As may best be seen in FIG. 9A,the plurality of electrically conductive sensing structures 41 includesa first set 41 a, a second set 41 b, and third set 41 c of sensingstructures arranged in rows 53 and columns 55 with spatially constantrow pitch and column pitch. Accordingly, the sensing structures 41 inthe fingerprint sensor according to the first example embodiment inFIGS. 9A-B exhibit a uniform sensing structure density (number ofsensing structures 41 per unit area) across the entire array. As isschematically indicated in FIGS. 9A-B, each sensing structure in thefirst set 41 a of sensing structures covers an area being within a firstarea range, each sensing structure in the second set 41 b of sensingstructures covers an area being within a second area range, and eachsensing structure in the third set 41 c of sensing structures covers anarea being within a third area range. The first area range includes afirst area A_(a) indicated in FIG. 9A, the second area range includes asecond area A_(b) indicated in FIG. 9A, and the third area rangeincludes a third area A_(c) indicated in FIG. 9A. The second area rangeonly includes areas greater than the areas within the first area range,and the third area range is between the first area range and the secondarea range.

Referring mainly to FIG. 9B, the first set 41 a of sensing structures isarranged to be covered by a thickness of the dielectric material 25within a first thickness range d₁-d₂, the second set 41 b of sensingstructures is arranged to be covered by thickness of the dielectricmaterial 25 within a second thickness range d₃-d₄, only includinggreater thicknesses than the first thickness range d₁-d₂, and the thirdset 41 c of sensing structures is arranged to be covered by thickness ofthe dielectric material 25 within a third thickness range d₂-d₃, betweenthe first thickness range and the second thickness range.

The capacitive coupling between a sensing structure 41 of thefingerprint sensor 23 and a finger 53 placed on the finger receivingsurface 51 is a measure of the capacitance of the capacitor formed bythe sensing structure 41, the finger 53, and the dielectric material 25between the sensing structure 41 and the finger surface. In theconfiguration in FIG. 9B, the maximum capacitance (resulting from aridge of the finger pattern in good contact with the finger receivingsurface 51) can be considered to be proportional to the surface area ofthe sensing structure 41 and inversely proportional to the distancebetween the sensing structure and the finger surface. To fullycompensate for differences in thickness of the dielectric material 25,the areas of the sensing structures 41 may thus vary continuouslyinversely with the thickness profile of the dielectric material 25.Accordingly, different sensing structures in the first set 41 a ofsensing structures may cover different areas, etc. This may bebeneficial for reducing or avoiding possible image artifacts resultingfrom sudden steps in sensing structure area. It should, however, benoted that the areas covered by the sensing structures may alternativelyvary step-wise. For instance, each sensing structure in the first set 41a of sensing structures may cover substantially the same area A_(a),each sensing structure in the second set 41 b may cover substantiallythe same area A_(b), and each sensing structure in the third set 41 bmay cover substantially the same area A_(c).

A third example embodiment of the fingerprint sensor 23 and fingerprintsensor arrangement 5 according to the present invention is schematicallyillustrated in FIGS. 10A-B. The second embodiment of the fingerprintsensor 23 in FIGS. 10A-B mainly differs from the first embodimentdescribed above with reference to FIGS. 9A-B in that the sensingstructures 41 are arranged in a non-uniform array configuration in thesensing plane 39. As can be seen in FIG. 10B, the sensing structures 41are arranged to exhibit a spatially varying sensing structure density(number of sensing structures per unit area in the sensing plane 39),that varies across the sensing plane depending on the non-uniformthickness profile of the dielectric material 25 to cover the sensingstructures 41. In particular, the sensing structure density is dependenton the rate of change of the thickness of the dielectric material 25, sothat the average sensing structure density is greater for the first set41 a of sensing structures than for the second set 41 b and the thirdset 41 c. With the exemplary non-uniform thickness profile for thefingerprint sensor arrangement 5 in FIG. 10B, the rate of change of thethickness of the dielectric material 25 is the lowest in the centralportion of the cross-section in FIG. 10B, and symmetrically increasestowards the edges.

As is schematically indicated by the capacitor symbols 61 a-c, thedirection of maximum capacitance for a particular sensing structure 41may not be normal to the sensing plane 39, but to the finger receivingsurface 51. Therefore, a gradually varying sensing structure density asindicated in FIGS. 10A-B may assist in correcting for image distortionresulting from the non-uniform thickness profile of the dielectricmaterial 25.

In embodiments, such as those described above with reference to FIGS.9A-B and 10A-B, where different sets 41 a-c of sensing structuresinclude sensing structures covering different areas in the sensing plane39, different configurations 57 a-c of the measurement circuitry 57 maybe used to amplify or adjust for the effects of the different sizes ofthe sensing structures 41. For example, the measurement circuitryconfigurations 57 a-c may be tuned to compensate for misalignment thatmay occur and that may cause spatial offset between the sensingstructure pattern and the non-uniform thickness profile of thedielectric material 25.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasured cannot be used to advantage.

What is claimed is:
 1. A fingerprint sensor to be included in afingerprint sensor arrangement comprising dielectric material having anon-uniform thickness profile defining a topography of a fingerreceiving surface of the fingerprint sensor arrangement, the fingerprintsensor comprising: a plurality of electrically conductive sensingstructures arranged in a sensing plane for capacitively sensing afingerprint of a finger placed on the finger receiving surface of thefingerprint sensor arrangement; and measurement circuitry coupled to theplurality of electrically conductive sensing structures for providingmeasurement signals indicative of a capacitive coupling between eachsensing structure in the plurality of electrically conductive sensingstructures and the finger, wherein the measurement circuitry is arrangedto: provide, using a first measurement circuitry configuration, a firstset of measurement signals from a first set of sensing structures in theplurality of electrically conductive sensing structures, arranged to becovered by a thickness of the dielectric material within a firstthickness range; and provide, using a second measurement circuitryconfiguration different from the first measurement circuitryconfiguration, a second set of measurement signals from a second set ofsensing structures in the plurality of electrically conductive sensingstructures, arranged to be covered by a thickness of the dielectricmaterial within a second thickness range, only including greaterthicknesses than the first thickness range.
 2. The fingerprint sensoraccording to claim 1, wherein the measurement circuitry is arranged toprovide the first set of measurement signals and the second set ofmeasurement signals during a fingerprint capture operation to allowformation of a fingerprint representation comprising the first set ofmeasurement signals and the second set of measurement signals.
 3. Thefingerprint sensor according to claim 2, wherein the measurementcircuitry is arranged to simultaneously provide the first set ofmeasurement signals and the second set of measurement signals.
 4. Thefingerprint sensor according to claim 1, wherein: the first measurementcircuitry configuration includes a first setting for at least one of anoffset and a gain of the measurement circuitry; and the secondmeasurement circuitry configuration includes a second setting, differentfrom the first setting, for at least one of the offset and the gain ofthe measurement circuitry.
 5. The fingerprint sensor according to claim1, wherein: the measurement circuitry comprises AD-converting circuitrycoupled to the plurality of electrically conductive sensing structures;the first measurement circuitry configuration includes a firstAD-converting circuitry configuration; and the second measurementcircuitry configuration includes a second AD-converting circuitryconfiguration, different from the first AD-converting circuitryconfiguration.
 6. The fingerprint sensor according to claim 5, wherein:the first AD-converting circuitry configuration includes a first settingfor at least one of an offset and a gain of the AD-converting circuitry;and the second AD-converting circuitry configuration includes a secondsetting, different from the first setting, for at least one of theoffset and the gain of the AD-converting circuitry.
 7. The fingerprintsensor according to claim 1, wherein: the measurement circuitrycomprises amplifier circuitry coupled to the plurality of electricallyconductive sensing structures; the first measurement circuitryconfiguration includes a first configuration for the amplifiercircuitry; and the second measurement circuitry configuration includes asecond configuration, different from the first configuration, for theamplifier circuitry.
 8. The fingerprint sensor according to claim 7,wherein the amplifier circuitry comprises a plurality of chargeamplifiers, each having a feedback capacitor.
 9. The fingerprint sensoraccording to claim 8, wherein the plurality of charge amplifierscomprises: at least a first charge amplifier coupled to the first set ofsensing structures, the first charge amplifier having a feedbackcapacitor with a first feedback capacitance; and at least a secondcharge amplifier coupled to the second set of sensing structures, thesecond charge amplifier having a feedback capacitor with a secondfeedback capacitance, different from the first feedback capacitance. 10.The fingerprint sensor according to claim 1, wherein: each sensingstructure in the first set of sensing structures covers an area in thesensing plane being within a first area range; and each sensingstructure in the second set of electrically conductive sensingstructures covers an area in the sensing plane being within a secondarea range only including areas greater than first area range.
 11. Afingerprint sensor arrangement, comprising: the fingerprint sensoraccording to claim 1; and dielectric material covering the sensing planeof the fingerprint sensor, the dielectric material having a non-uniformthickness profile defining the topography of the finger receivingsurface of the fingerprint sensor arrangement.
 12. A fingerprint sensingsystem comprising: a fingerprint sensor arrangement according to claim11; and a fingerprint sensor controller coupled to the fingerprintsensor for controlling operation of the fingerprint sensor.
 13. Afingerprint sensing system according to claim 12, wherein thefingerprint sensor controller is configured to: control the measurementcircuitry of the fingerprint sensor to provide initial measurementsignals using an initial set of measurement circuitry configurations;determine, based on the initial measurement signals, the firstmeasurement circuitry configuration and the second measurement circuitryconfiguration; and control the measurement circuitry to provide thefirst set of measurement signals using the first measurement circuitryconfiguration and the second set of measurement signals using the secondmeasurement circuitry configuration.
 14. An electronic devicecomprising: a device housing with a convex portion having an opening;and the fingerprint sensor arrangement according to claim 11 arranged inthe opening of the curved portion of the device housing.
 15. A method ofacquiring a fingerprint representation using a fingerprint sensorarrangement with a finger receiving surface to be touched by a finger,the fingerprint sensor arrangement comprising a plurality ofelectrically conductive sensing structures arranged in a sensing plane;measurement circuitry coupled to the plurality of electricallyconductive sensing structures for providing measurement signalsindicative of a capacitive coupling between each sensing structure inthe plurality of electrically conductive sensing structures and thefinger; and dielectric material covering the sensing plane, thedielectric material having a non-uniform thickness profile defining atopography of the finger receiving surface of the fingerprint sensorarrangement, the method comprising the steps of: acquiring, using afirst measurement circuitry configuration, a first set of measurementsignals from a first set of sensing structures in the plurality ofelectrically conductive sensing structures, covered by a thickness ofthe dielectric material within a first thickness range; and acquiring,using a second measurement configuration setting different from thefirst measurement circuitry, a second set of measurement signals from asecond set of sensing structures in the plurality of electricallyconductive sensing structures, covered by a thickness of the dielectricmaterial within a second thickness range, only including greaterthicknesses than the first thickness range.